Breathe in the lower part of your lungs. Then gradually use your diaphragm and keep your core muscles in decreasing-balloon-size mode as you breathe out (sing/exhale/blow) with gradual pressure, controlling. Keep your chest and shoulders relaxed all the time. Concentrate on your core.
Watch the whole set of exercises on this video (this was early morning in the lovely forest where I was brought up, so my voice had a few unwanted glitches that cleared up as the day went by): https://youtu.be/SfDR57gOY_4